It is naturally sensum communem that the Liegeois would not stamp the anchor or any other symbol in the lock plate of a generic (mass) production. What i have (only) suggested is, if the pistol in the museum was part of a private purchase, which all indicates it is, it would be nothing implausible that the client, a ship's captain or a sailing enterprise, would order a lot of such fireamrs for his/their crew and send along the technical drawing for the anchor to be engraved in the origin workshops. Otherwise, it is left to know where the client took the gun/s go be engraved elsewhere in England; he would certainly not require the job from the BO facilities.
Just a pitty the museum photo doesn't have macro opitions to visualize both anchor and the two initials with one's eye.
My humble perspective, this is
.